Jackshaft garage door openers that lift the door by turning the counterbalance shaft have been known by those skilled in the art for quite some time. Jackshaft garage door openers are primarily used on sectional doors with lift clearance, or full vertical, style track configurations since a portion, or all, of the door remains in the vertical orientation when the door is open. When closing, the jackshaft opener turns the counterbalance assembly and winds the counterbalance springs while paying out cable. The door is lowered by the weight of the portion of the door in the generally vertical position, relative to the ground, applying a downward force to the remainder of the door that is in the generally horizontal position, relative to the ground. This downward force also keeps the cables tensioned as the door is closed.
Sectional doors are moveable barriers used to secure an opening in a wall or structure. The opening is usually comprised of a header which is parallel, relative to the ground, spanning the very top of the opening, a floor at the very bottom of the opening which is parallel relative to the ground, and side jambs which are normal, relative to the ground, and span the left and right side of the opening from the floor to the header. The sectional door is in the closed position when the bottom section of the door is in contact with the floor and the entire opening is secured by the sectional door blocking the opening. The sectional door is considered to be in the open position when the very lowest portion, relative to the ground, of the bottom section is near the header of the opening allowing entry and exit through the opening.
On standard lift sectional doors near, or in, the open position, very little of the door, if any, is in the vertical position relative to the ground. Turning the counterbalance assembly to close a standard lift door from near, or at, the open position where insufficient door weight is in the vertical orientation, relative to the ground, leads to a situation where the cables could become un-tensioned and unwrap from the cable drums. Cables which become unwrapped from a cable drum result in an unsafe condition in which the door could drop uncontrollably. A further complication of cables coming unwrapped is the inability to lift the door without binding cables around the counterbalance shaft and potentially breaking the cable and allowing the door to drop to the ground uncontrollably. Cables can also lose tension and unwrap from cable drums in the event a door binds or encounters an obstruction while it is closing. This could occur not only on standard lift doors, but also on lift clearance and full vertical sectional doors.
Over the years slack cable sensors of various designs have been used to detect a loss of cable tension on sectional doors. These sensors include mechanical and electrical versions all with the same intended purpose, to stop the door from closing when the lift cables are un-tensioned and could potentially unwrap from the cable drums. Electrical versions are connected to inputs on motorized operators to alert the motorized operator that cables are un-tensioned and to stop. Slack cable sensors complicate the installation of a jackshaft opener by requiring additional equipment and installation time. They also cannot prevent the cables from slacking, but rather only detect it.
Standard lift doors can sometimes be modified to increase the amount of force acting in the vertical position when the door is in the open position. Those skilled in the art should be familiar with modifying the horizontal tracks of standard lift doors to provide some vertical lift and/or installing pusher springs on the back of the horizontal tracks to push the door closed for brief amount of travel from the open position. Both of these modifications require additional time and equipment, and should be done only by a highly trained individual. Another method of attempting to provide a closing force to standard lift doors when closing via a jackshaft opener has been the addition of one or more cable drums to the door itself on the counterbalance shaft to take up cables attached to the top section of the door to pull it closed. The problem with this method is that as sectional door is closing from the open to close position the top of the door transitions from the horizontal to the vertical position and the top edge of the door moves closer to and then further away from the cable drums. This creates a situation where the cable drums pulling the door closed take up cable and then have to pay out cable during the closing operation. The counterbalance shaft rotates in only one direction as the door closes and thus does not allow for a cable attached to the top of the door to be taken up and then paid out as the top of the door transitions from the horizontal to the vertical orientation. Several devices have been previously proposed to address this. One such prior art device is disclosed in U.S. Pat. No. 4,191,237 which describes an operator used to rotate a counterbalance assembly with cable drums to take up cable thereby lifting the door and another cable drum on the counterbalance assembly that is used to pull the door closed. In order to address the transitioning of the top section from horizontal to vertical the '237 patent discloses a fixed pulley mounted below the counterbalance assembly and an additional pulley attached to a bracket mounted to the top of the door. These inconvenient modifications require additional cost of equipment and time to install and add undesirable complexity to the door system.
Another mechanism disclosed in U.S. Pat. No. 6,883,579 also includes a cable to pull the door closed. The cable is connected to the top section of the door through an arm bracket at one end and to a cable drum on the counterbalance shaft at the opposite end. The point of attachment of the cable to the arm bracket remains in the horizontal position throughout the opening/closing operation. The modification of the door to add the arm bracket and the need for a longer horizontal length of door track is costly, time consuming to install and, therefore, undesirable.
U.S. Pat. No. 6,326,751 also describes a cable that spans between and connects the top of the door to a cable drum mounted on the door counterbalance shaft. The '751 patent describes attachment of the upper cable to the door utilizing a tension member such as an extension spring. While closing, the top section of the door starts transitioning from a generally horizontal to generally vertical orientation, relative to the ground, at which point the top section of the door begins to move away from the cable drum thereby stretching the extension spring while the door continues to close. The problem with this device is the spring, while flexible enough to allow it to wrap on the drum, is actually trying to pull the door open, not closed once the top section of the door has transitioned from horizontal to vertical. In order for the spring to be flexible enough to wrap on the drum it also cannot provide any significant tensile force to the top of the door when pulling it closed from the open position rendering the application of this device impractical in reality.
Consequently, there is a long felt need in the art for a jackshaft opener that not only opens a door by rotating the counterbalance shaft and cable drums to take up lift cables attached to the bottom of the door, but will also provide a force applied to the door so as to positively drive a door closed without relying on the weight of a portion of the door hanging in the vertical position, relative to the ground, and without relying on the addition of costly equipment or time consuming modification of the door. There is also a long felt need in the art for a jackshaft opener that can directly sense the position of the door so as to determine whether or not the door is moving while the counterbalance assembly is turned so as to detect slack cables without additional equipment or modification of the door. Finally, there is a long felt need in the art for a jackshaft opener that accomplishes all of the forgoing objectives, and that is relatively inexpensive to manufacture and safe and easy to use.